Silver alloy sheathing material for ceramic superconducting wires

ABSTRACT

The invention relates to a sheathing material for superconducting wires which are deformed during manufacture by drawing or a similar procedure. The superconducting material of the wires is composed of an oxide whose superconducting properties worsen during the deformation so that, in order to recover its original superconducting properties or to further improve them, the material must be subjected to a recovery heat treatment at temperatures above 940° C. 
     Customarily, silver is employed as the sheathing material for such wires. The recovery heat treatment is generally performed at temperatures around 900° C. Experiments have shown that the optimum temperature range for a recovery heat treatment lies between about 940° C. and 1030° C. However, these temperatures were above the melting temperature of silver in an oxygen atmosphere. It is the object of the invention to find a sheathing material which has the favorable properties of silver but melts only at a temperature above the optimum temperature for the recovery heat treatment. 
     This is accomplished according to the invention in that the sheathing material employed is a silver alloy whose melting point lies above the melting point of pure silver.

FIELD OF THE INVENTION

The invention relates to a sheathing material for superconducting wiresaccording to the preamble of claim 1.

Technology Review

In technologically usable superconducting wires, the superconductingoxide must be surrounded by a metal sheath. This metallic sheathingmaterial performs several important tasks.

One function of the sheathing material is to minimize heating as theresult of local losses of the superconducting property of the oxide andthus contributes to the retention of the superconductive property of theentire unit when the transition temperature is exceeded. Anotherfunction of the sheathing is to support the brittle superconductingoxide against the usually unavoidable mechanical stresses which occurduring its manufacture and also as a result of the Lorentz force duringoperation of a superconductor.

Moreover, the sheathing material must be able to air-tightly seal thesuperconducting core against the atmosphere at temperatures below 300°K. so that its oxygen content remains unchanged during storage as wellas during operation.

All prior art high field superconductors at B_(C2) >15 Tesla aredistinguished by extraordinary brittleness. This property quitesignificantly influences the manufacturing process for wires. Wirescontaining high field superconductors such as YBa₂ Cu₃ O₇ or oxidesderived from it (substitution of Y by rare earths) can at present bemanufactured only by powder technology methods.

In this case, the already formed (pre-reacted) superconducting phase isfilled into a metal tube, is compressed and shaped by deformationprocesses such as hammering, rolling or drawing into a thin wire havinga diameter of about 1 mm.

It is not yet known how the deformation process takes place in detailwithin the superconducting phase, but it has been possible to detect theresults of the deformation by measuring the transition temperature T_(c)and the critical current density J_(c).

Although for YBa₂ Cu₃ O₇ and oxides derived therefrom T_(c) remainsconstant, the percentage of the superconducting phase and the criticalcurrent density J_(c) decrease with increasing degree of deformation.The superconducting properties disappear completely if the deformationis extensive. To recover the original superconducting properties at alater time, recovery heat treatments are necessary which are customarilyperformed at temperatures above 800° C., frequently at about 900° C. toa maximum of 940° C., which is followed by an oxidation heat treatment.

In the past, the sheathing material for the production of high fieldsuperconducting wires has almost exclusively been pure silver (Proc. 1.European Workshop on High T_(c) Superconductors and PotentialApplications, P. Dubots et al, page 133; G. Barani et al, page 137; R.Flukiger et al, page 131; and S. Jin et al, Appl. Phys. Lett. 51 (1987),page 203).

Pure silver is well suited for this purpose for several reasons. It isductile, its melting point in air lies at 960.8° C. and in a pure oxygenatmosphere at 939° C.; it does not undergo a reaction with the oxidicsuperconductor if it is heat treated and finally it is permeable tooxygen at temperatures above 400° C. From an economic point of view, theprice of silver is considered to be still acceptable for the intendedpurpose.

Although Jin et al propose a sheathing material of copper including adiffusion barrier of nickel/gold, copper is less suitable due to itsirreversible formation of copper oxide.

For a superconductor of the YBa₂ Cu₃ O₇ type, the above mentionedrecovery heat treatment is usually performed at temperatures barelybelow the melting point of silver in an oxygen atmosphere. This recoveryheat treatment is then followed by an oxidation heat treatment at about400° C. to 700° C. at which the oxygen loss in the oxidicsuperconducting material YBa₂ Cu₃ O₇ --which is about 2% at 900° C.--isreplaced again.

SUMMARY OF THE INVENTION

Our tests have shown that the critical current density ofsuperconducting YBa₂ Cu₃ O₇ wires can be increased substantially if therecovery heat treatment is not performed at 900° C., with a maximum at939° C., but at temperatures in a range between 940° C. and 1030° C.,that is, in a temperature range in which silver already melts in anoxygen atmosphere.

It is thus an object of the invention to provide sheathing material forsuperconducting wires which has at least the same, above-mentioned,favorable properties as silver but which melts at a higher temperatureso that, with this sheathing material, the recovery heat treatment canbe performed in the optimum temperature range at temperatures higherthan 940° C. up to 1030° C.

The cost of this sheathing material does not significantly exceed theprice of silver.

The silver alloy employed as the inventions sheathing material hasmelting point above the melting point of pure silver. Such a silveralloy can be produced by alloying at least one of the elements from thegroup including gold, palladium, platinum, manganese and titanium withsilver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following table gives examples of silver alloys within the scope ofthe invention which have a melting point of at least 1000° C.

                  TABLE                                                           ______________________________________                                        Alloy       Composition* Melting Point                                        ______________________________________                                        Ag--Au      35    weight % Au                                                                              1000° C.                                  Ag--Mn      23    weight % Mn                                                                              1000° C.                                              31    weight % Mn                                                                              1071° C.                                  Ag--Pd      10    weight % Pd                                                                              1000° C.                                              20    weight % Pd                                                                              1070° C.                                  Ag--Pt      20    weight % Pt                                                                              1000° C.                                              35    weight % Pt                                                                              1050° C.                                  Ag--Ti      3     weight % Ti                                                                              1017° C.                                  ______________________________________                                         *Difference compared to 100 weight % Ag                                  

The above listed favorable properties of pure silver are not negativelyinfluenced by the admixture of an alloying component from theabove-mentioned group.

As indicated by the examples listed below, a higher recovery heattreatment temperature considerably increase the value of the criticalcurrent density J_(c). However, the recovery heat treatment temperaturecan only be raised to values above the previously employed temperatureof about 900° C., with a maximum at 939° C., if a sheathing material isemployed which has the favorable properties of silver but whose meltingpoint is higher than the temperature at which the recovery heattreatment must be performed in order to obtain optimum results.

The invention will now be described in greater detail with reference toproduction examples.

EXAMPLE 1

A superconducting wire was produced by drawing of a tube (12×1.5 mm)composed of a silver-palladium alloy containing 8 weight percent Pd andfilled with YBa₂ Cu₃ O₇ powder. After drawing, the sheathing materialhad an external diameter of 1.15 mm and an internal diameter of 0.85 mm.

The recovery heat treatment was performed at a temperature of 970° C.for a heat treatment duration of 80 minutes. Then an oxidation heattreatment was performed in which a temperature of 680° C. was maintainedfor six hours and a temperature of 550° C. was maintained for 48 hours.Both heat treatments were performed in an oxygen atmosphere. Thesuperconducting wire produced in this manner attained a critical currentdensity of 450 A cm² (with reference to its cross-section). This valueis noticeably higher than that of J_(c) =175 A/cm² published by Jin etal which was realized with a sheathing material made of silver andrecovery and oxidation heat treatment temperatures of 900° C. and 600°C., respectively.

EXAMPLE 2

A superconducting wire was produced by drawing a tube (12×1 mm) made ofa silver-palladium alloy containing 20 weight percent Pd and filled withYBa₂ Cu₃ O₇ powder. After drawing, the sheathing material had anexternal diameter of 0.4 mm and a wall thickness of 0.03 mm.

The recovery heat treatment was performed at a temperature of 990° C.for a heat treatment duration of 75 minutes. Then an oxidation heattreatment was performed at three different temperature stages:

1. 6 hours at 680° C;

2. 24 hours at 550° C.;

3. 48 hours at 450° C.

Both heat treatments were performed in an oxygen atmosphere.

The superconducting wire produced in this way attained a criticalcurrent density of 680 A/Cm² (with reference to its cross section).

Other modifications and variation of the present invention are possiblein light of the above teachings. It is, therefore, to be understood thatchanges may be made in the particular embodiments described above whichare within the full intended scope of the invention as defined in theappended claims.

I claim:
 1. A sheathing material for superconducting wires which aredeformed during production by drawing or a similar process, and whichare made of a superconducting material comprising an oxide havingsuperconducting properties which worsen during deformation and whichmust be subjected, in order to recover said superconducting propertiesor to improve said superconducting properties, to a recovery heattreatment before a later oxidation heat treatment, wherein saidsheathing material comprisesa silver alloy having a melting point abovethe melting point of pure silver.
 2. Sheathing material according toclaim 1, wherein said silver alloy comprises silver alloyed with atleast one of the elements selected from the group consisting of gold,palladium, platinum, manganese and titanium.
 3. Sheathing materialaccording to claim 2, wherein the concentration of the elements alloyedin said silver alloy is selected so that the melting point of saidsheathing material is above the optimum temperature for a recovery heattreatment.
 4. A superconductor comprising:a sheathing material forsuperconducting wires which are deformed during production by drawing ora similar process, and which are made of a superconducting materialcomprising an oxide having superconducting properties which worsenduring deformation and which must be subjected, in order to recover saidsuperconducting properties or to improve said superconductingproperties, to a recovery heat treatment before a later oxidation heattreatment, said sheathing material having an interior and comprising asilver alloy having a melting point above the melting point of puresilver; and a superconductive material disposed in said interior of saidsheathing material, said superconductive material and said sheathingmaterial being jointly deformed, and then being jointly subjected to arecovery heat treatment so that said superconducting properties arerecovered.
 5. A superconductor as defined in claim 4, wherein saidsuperconductive material comprises a powdered oxide.